(in-package :cl-user)

(defvar bar nil)

(defun foo (&key bar)
  42)


#|
CL-USER> (foo :bar 2)

Debugger invoked on condition of type PROGRAM-ERROR:
  Unrecognized keyword argument :BAR

...

which is a little weird, because the lambda list for FOO looks ok:

CL-USER>  (describe #'foo)
#<FUNCTION FOO {5B663F}> is an object of type COMPILED-FUNCTION.
The function's lambda list is:
  (&KEY BAR)

|#